The stock of Icahn Enterprises LP IEP tumbled 21% Thursday, a day after billionaire hedge-fund manager Bill Ackman, founder and chief executive of Pershing Square Capital Management, revived his longstanding feud with Carl Icahn in commentary on a recent short seller report. That report by Hindenburg Research accused Icahn’s publicly traded investment vehicle of inflating asset values and and causing his company to trade at a large premium. The report has cost IEP some $10.9 billion in lost market cap including Thursday’s move.